Conventional electrodeionization apparatus has a structure in which a plurality of cation exchange membranes and a plurality of anion exchange membranes are alternately arranged between electrodes (anode and cathode) in such a manner as to alternately form desalting compartments and concentrating compartments and the desalting compartments are filled with ion exchange resin. In the electrodeionization apparatus, water to be treated is introduced into the desalting compartments and concentrated water is introduced into the concentrating compartments while voltage is applied between the cathode and the anode so that impurity ions are removed from the water to be treated, thereby producing deionized water.
Since the desalting compartments and the concentrating compartments are alternately formed between the cathode and the anode in the conventional electrodeionization apparatus, electric resistance between the cathode and the anode is large and the voltage applied between the cathode and the anode is high. In addition, calcium carbonate scale is sometimes generated on ion exchange membranes in the concentrating compartments due to Ca2+ and components (CO2, HCO3−) in raw water.
The first object of the present invention is to provide an electrodeionization apparatus which is suitably employed in case of small amount of deionized water as product water and in which the voltage applied between electrodes is low and scale is hardly generated.